Low-crosstalk electro-optical mach-zehnder switch

ABSTRACT

Optical switches and methods of switching include a first hybrid coupler configured to accept an input and to provide two branches. A phase tuner on a first branch includes a Mach-Zehnder phase shifter configured to shift a signal on the first branch by a selected phase. A loss compensator on a second branch is configured to match a loss incurred on the first branch. A second hybrid coupler is configured to recombine the two branches such that the phase shift generated by the phase tuner determines which output of the second hybrid coupler is used.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a Continuation application of co-pending U.S. patentapplication Ser. No. 14/686,226, filed on Apr. 14, 2015, incorporatedherein by reference in its entirety.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

This invention was made with Government support under Contract No.W911NF-12-2-0051 awarded by Defense Advanced Research Projects Agency(DARPA). The Government has certain rights to this invention.

BACKGROUND

Technical Field

The present invention relates to optical switching and, moreparticularly, to optical switches that exhibit low crosstalk betweenoutputs.

Description of the Related Art

Integrated 2×2 Mach-Zehnder interferometers can be used as basicelements of large transparent optical switches. An input signal is splitin a 3-dB input coupler along two branches and the branches aresubsequently recombined at a 3-dB output coupler. A phase controller isused on each branch to control how the signals on the branches arerecombined, resulting in an output along either the first or the secondbranch of the output coupler. When the phase difference between the twobranches is (2n+1)π, the first branch is selected, and when the phasedifference is 2nπ, the second branch is selected.

In reality, however, there is usually some light leakage at thenon-selected output that can be due to power imbalance in theinterferometer, phase errors (where the phase controllers do notproduce, for example, perfect integer multiples of pi), and imperfectcouplers. This crosstalk puts a limit on the effectiveness of theswitch, as less than full power is transmitted along the selected outputand a potentially significant signal is present on the non-selectedoutput.

One existing solution is to use a thermo-optic phase shifter on onebranch that has a phase range from zero to pi. This can achieve a lowcrosstalk, but switching speeds are on a millisecond-scale, whereasoptical switching networks could benefit from nanosecond-scaleswitching.

Electro-optical phase shifters using carrier injection in a pin diode,where a phase shift is obtained by modulating the carrier density. Thiscan achieve nanosecond-scale switching rates, but carrier injectioncreates optical losses due to free-carrier absorption. A power imbalanceresults, affecting the crosstalk. Such a system produces, at best, −20dB of isolation. This result can be improved somewhat by using push-pulldrive configuration, with one phase shifter on each branch having aphase range of zero to one-half pi, but this produces only a moderatebenefit and an isolation of about −25 dB.

As such, no existing solution can produce nanosecond-scale switching anda suitably low crosstalk.

SUMMARY

An optical switch includes a first hybrid coupler configured to acceptan input and to provide two branches. A phase tuner on a first branchincludes a Mach-Zehnder phase shifter configured to shift a signal onthe first branch by a selected phase. A loss compensator on a secondbranch is configured to match a loss incurred on the first branch. Asecond hybrid coupler is configured to recombine the two branches suchthat the phase shift generated by the phase tuner determines whichoutput of the second hybrid coupler is used.

A system for optical switching includes an optical switch and a controlmodule. The optical switch includes a first branch comprising aMach-Zehnder phase shifter and a first heater and a second branchcomprising a loss compensator configured to match a loss incurred on thefirst branch and a second heater. The control module is configured toselect a phase shift in the Mach-Zehnder phase shifter to determine anoutput of the optical switch and to activate the first or second heaterto reduce crosstalk between outputs of the optical switch.

A method for optical switching includes setting a phase shift in a firstbranch of an optical switch. A loss compensation in a second branch ofthe optical switch is set to match a loss incurred on the first branch.A crosstalk at an output of the optical switch is measured and, if themeasured crosstalk exceeds a crosstalk threshold, a heater is activatedon the first or second branch to reduce the crosstalk.

These and other features and advantages will become apparent from thefollowing detailed description of illustrative embodiments thereof,which is to be read in connection with the accompanying drawings.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

The disclosure will provide details in the following description ofpreferred embodiments with reference to the following figures wherein:

FIG. 1 is a block diagram of a low-crosstalk optical switch inaccordance with the present principles;

FIG. 2 is a block diagram of a phase shifter used in a low-crosstalkoptical switch in accordance with the present principles;

FIG. 3 is a block diagram of a low-crosstalk optical switch inaccordance with the present principles;

FIG. 4 is a block diagram of a control module for a low-crosstalkoptical switch in accordance with the present principles;

FIG. 5 is a block/flow diagram of switching control in accordance withthe present principles; and

FIG. 6 is a block/flow diagram of switching control in accordance withthe present principles.

DETAILED DESCRIPTION

To achieve low crosstalk at high switching speeds, the present inventionprovides a 2×2 Mach-Zehnder (MZ) switch having a phase tuning mechanismand a loss compensating mechanism. A nested design is used in oneembodiment, with a first MZ switch implemented on one branch of a secondMZ switch.

Referring now to the drawings in which like numerals represent the sameor similar elements and initially to FIG. 1, a diagram of thelow-crosstalk MZ switch 100 is shown. A first 2×2 coupler 102 acceptstwo inputs from, e.g., respective waveguides. These inputs representsignals that are coupled in the coupler 102 to produce two mixed signalsalong the outputs of the coupler 102. A phase tuner 104 is implementedon the first output of the coupler 102, while a loss compensator 106 isimplemented on the second output of the coupler 102. A second 2×2coupler 108 is then used to combine the “internal” branches of the MZswitch. Depending on the relative phase of the two branches, either afirst output or a second output waveguide is used to provide the outputsignal. It is specifically contemplated that the couplers 102 and 108are 2×2 3 dB hybrid couplers that split the input power equally betweentwo output branches. It should be understood that, while the presentstructures are shown as implementing a four-way optical switch, thepresent principles may be used to implement an optical switch having anycombination of M input ports and N output ports.

One embodiment includes a phase shifter 104 implemented with a MZ phaseshifter driven in push-pull for digital phase modulation. The losscompensator 106, meanwhile, can be a fixed or tunable attenuator. Animplementation of a tunable attenuator could be a forward biased pindiode, also called a variable optical attenuator (VOA). If insertionlosses of the phase shifter 104 are known, the loss compensator 106 canalso be implemented by implanting dopants into a short section of thewaveguide or using reflection elements or scattering elements.

It is specifically contemplated that the present embodiments may beimplemented using optical waveguides on a silicon substrate, but anyappropriate materials may be used. In particular, some alternativesinclude SiN, SiON, SiO₂, InP, GaAs, LiNbO₃, and polymer materials.

Referring now to FIG. 2, a diagram of a MZ phase shifter 200 is shown,which may be used as an implementation of the phase tuner 104. A singleinput is provided to a first 2×2 coupler 202, with the other input beingleft unconnected. The coupler 202 splits the single input into twobranches. A first branch has a first phase shifter 204 and a secondbranch has a second phase shifter 206. This embodiment uses a push-pullconfiguration, where each of the phase shifters have a range of zero topi and operate in opposition to one another. The branches are coupledagain at a second 2×2 coupler 208, where a single, phase-shifted outputis taken as the output of the MZ phase shifter 200.

Assuming perfect 3 dB couplers are used for couplers 202 and 208,assuming phase shifters with no insertion loss, and assuming normalizedinput, the field at the output of the MZ phase shifter 200 at a phaseshift Δφ is:

${T\left( {\Delta \; \varphi} \right)} = {^{\frac{j{({\varphi_{1} + \varphi_{2}})}}{2}}{{\sin \left( \frac{\Delta \; \varphi}{2} \right)}.}}$

where φ₁ and φ₂ are phases selected by the respective phase shifters 204and 206 and where Δφ is the difference between φ₁ and φ₂.

When driving the phase shifters 204 and 206 in a push-pull fashion withΔφ=±π, the field at the output of the phase shifter 200 simplifies to:

T(±π)=±j,

producing a digital phase shift. If the insertion loss of the phaseshifter 200 is α, then the output field is:

T(±π)=±j(1−α).

The lower branch of the switch 100 therefore needs to match this loss.The loss tuner 106 is, for example, a VOA implemented as a forwardbiased pin diode. The output of the loss compensator 106 can be:

T=1−α,

Assuming perfect couplers and perfect drive, the present embodimentsprovide exemplary crosstalk suppression of about −90 dB.

Referring now to FIG. 3, an alternative embodiment of a nested MZ switch300 is shown that uses a feedback loop to correct phase and amplitudeerrors. The switch 300 uses phase detectors 302 to measure phaseperformance. In addition, two heaters 304 and 306 are used to correctphase errors that are present inside the interferometer. The heaters maybe implemented using any appropriate technology, including resistiveheating and thermoelectric effect heating. Because phase error isrelatively stable compared to the switching speed, heaters aresufficient to correct for phase drift.

The photodetector 302 at the output of the phase tuner 104 measures thesquare of the phase error. The feedback loop thus tests both heaters 304and 306 to pick the heater than minimizes the crosstalk, as measured byphotodetectors 303 at the output, where the photodetectors 303 tap about1% of the power of the optical output to measure the power and crosstalkof the outputs. A control module 308 collects phase information from thephotodetector 302 and power and crosstalk information fromphotodetectors 303 and determines which heater to use. Based on thephase information collected by the photodetector 302 and the crosstalkinformation collected by photodetectors 303, the control module 308activates either a first heater 306 or a second heater 304 to correctthe phase error.

Fabrication imperfections that lead to different doping levels inoptical circuit components, slightly different phase shifter lengths,different waveguide roughness, and other factors can introduceadditional phase error in the switch 100 and thus increase the level ofcrosstalk. For example, if the phase difference produced by the phasetuner 200 is Δφ≠±π, the transmission of an MZ phase tuner 200 will be:

${{T\left( {\Delta \; \varphi} \right)} = {^{\frac{j{({\varphi_{1} + \varphi_{2}})}}{2}}{\sin \left( \frac{\Delta \; \varphi}{2} \right)}\left( {1 - \alpha} \right)}},$

and the phase error is:

$\varepsilon_{\varphi} = {\frac{\pi}{2} - {\frac{\varphi_{1} + \varphi_{2}}{2}.}}$

To compensate for this phase error, the present embodiments add either−ε_(φ) to the upper arm or add ε_(φ) to the lower arm. Assuming that thephase error is small, the power at the second output of the phase tuner200—the output that reaches a photodetector 302—is approximated as:

$P_{l} = {{T}^{2}\text{∼}\frac{\left( {1 - \alpha} \right)^{2}\varepsilon_{\varphi}^{2}}{4}}$

with a phase error of:

$\varepsilon_{\varphi} \approx {\pm {\frac{2\sqrt{P_{l}}}{1 - a}.}}$

The phase error can then be detected by the photodetector 302 at thelower output port of the phase tuner 200.

The present invention may be a system, a method, and/or a computerprogram product. The computer program product may include a computerreadable storage medium (or media) having computer readable programinstructions thereon for causing a processor to carry out aspects of thepresent invention.

The computer readable storage medium can be a tangible device that canretain and store instructions for use by an instruction executiondevice. The computer readable storage medium may be, for example, but isnot limited to, an electronic storage device, a magnetic storage device,an optical storage device, an electromagnetic storage device, asemiconductor storage device, or any suitable combination of theforegoing. A non-exhaustive list of more specific examples of thecomputer readable storage medium includes the following: a portablecomputer diskette, a hard disk, a random access memory (RAM), aread-only memory (ROM), an erasable programmable read-only memory (EPROMor Flash memory), a static random access memory (SRAM), a portablecompact disc read-only memory (CD-ROM), a digital versatile disk (DVD),a memory stick, a floppy disk, a mechanically encoded device such aspunch-cards or raised structures in a groove having instructionsrecorded thereon, and any suitable combination of the foregoing. Acomputer readable storage medium, as used herein, is not to be construedas being transitory signals per se, such as radio waves or other freelypropagating electromagnetic waves, electromagnetic waves propagatingthrough a waveguide or other transmission media (e.g., light pulsespassing through a fiber-optic cable), or electrical signals transmittedthrough a wire.

Computer readable program instructions described herein can bedownloaded to respective computing/processing devices from a computerreadable storage medium or to an external computer or external storagedevice via a network, for example, the Internet, a local area network, awide area network and/or a wireless network. The network may comprisecopper transmission cables, optical transmission fibers, wirelesstransmission, routers, firewalls, switches, gateway computers and/oredge servers. A network adapter card or network interface in eachcomputing/processing device receives computer readable programinstructions from the network and forwards the computer readable programinstructions for storage in a computer readable storage medium withinthe respective computing/processing device.

Computer readable program instructions for carrying out operations ofthe present invention may be assembler instructions,instruction-set-architecture (ISA) instructions, machine instructions,machine dependent instructions, microcode, firmware instructions,state-setting data, or either source code or object code written in anycombination of one or more programming languages, including an objectoriented programming language such as Smalltalk, C++ or the like, andconventional procedural programming languages, such as the “C”programming language or similar programming languages. The computerreadable program instructions may execute entirely on the user'scomputer, partly on the user's computer, as a stand-alone softwarepackage, partly on the user's computer and partly on a remote computeror entirely on the remote computer or server. In the latter scenario,the remote computer may be connected to the user's computer through anytype of network, including a local area network (LAN) or a wide areanetwork (WAN), or the connection may be made to an external computer(for example, through the Internet using an Internet Service Provider).In some embodiments, electronic circuitry including, for example,programmable logic circuitry, field-programmable gate arrays (FPGA), orprogrammable logic arrays (PLA) may execute the computer readableprogram instructions by utilizing state information of the computerreadable program instructions to personalize the electronic circuitry,in order to perform aspects of the present invention.

Aspects of the present invention are described herein with reference toflowchart illustrations and/or block diagrams of methods, apparatus(systems), and computer program products according to embodiments of theinvention. It will be understood that each block of the flowchartillustrations and/or block diagrams, and combinations of blocks in theflowchart illustrations and/or block diagrams, can be implemented bycomputer readable program instructions.

These computer readable program instructions may be provided to aprocessor of a general purpose computer, special purpose computer, orother programmable data processing apparatus to produce a machine, suchthat the instructions, which execute via the processor of the computeror other programmable data processing apparatus, create means forimplementing the functions/acts specified in the flowchart and/or blockdiagram block or blocks. These computer readable program instructionsmay also be stored in a computer readable storage medium that can directa computer, a programmable data processing apparatus, and/or otherdevices to function in a particular manner, such that the computerreadable storage medium having instructions stored therein comprises anarticle of manufacture including instructions which implement aspects ofthe function/act specified in the flowchart and/or block diagram blockor blocks.

The computer readable program instructions may also be loaded onto acomputer, other programmable data processing apparatus, or other deviceto cause a series of operational steps to be performed on the computer,other programmable apparatus or other device to produce a computerimplemented process, such that the instructions which execute on thecomputer, other programmable apparatus, or other device implement thefunctions/acts specified in the flowchart and/or block diagram block orblocks.

The flowchart and block diagrams in the Figures illustrate thearchitecture, functionality, and operation of possible implementationsof systems, methods, and computer program products according to variousembodiments of the present invention. In this regard, each block in theflowchart or block diagrams may represent a module, segment, or portionof instructions, which comprises one or more executable instructions forimplementing the specified logical function(s). In some alternativeimplementations, the functions noted in the block may occur out of theorder noted in the figures. For example, two blocks shown in successionmay, in fact, be executed substantially concurrently, or the blocks maysometimes be executed in the reverse order, depending upon thefunctionality involved. It will also be noted that each block of theblock diagrams and/or flowchart illustration, and combinations of blocksin the block diagrams and/or flowchart illustration, can be implementedby special purpose hardware-based systems that perform the specifiedfunctions or acts or carry out combinations of special purpose hardwareand computer instructions.

Referring now to FIG. 4, a block diagram of a control module 400 isshown. The control module 400 includes a processor 402 and memory 404.The processor 402 interacts with a set of different modules, each ofwhich may be implemented as software, as hardware in the form of anapplication-specific integrated chip or field programmable gate array,or as a combination of software and hardware modules. The memory 404stores phase information and setting information for phase and losscontrol.

A phase detection module 405 receives phase information from the phasedetectors 302 to determine the phase error inside the interferometer. Aphase control module 406 interfaces with the phase tuner 104 to controlthe phase of the signal passing through that branch. A loss controlmodule 408, meanwhile, interfaces with the loss compensator 106 tocontrol the loss passing through its respective branch to match the lossof the phase-controlled branch. Heater control modules 410 and 412interface with respective heaters 304 and 306 to trigger heating of onebranch or the other to correct for phase differences.

Reference in the specification to “one embodiment” or “an embodiment” ofthe present principles, as well as other variations thereof, means thata particular feature, structure, characteristic, and so forth describedin connection with the embodiment is included in at least one embodimentof the present principles. Thus, the appearances of the phrase “in oneembodiment” or “in an embodiment”, as well any other variations,appearing in various places throughout the specification are notnecessarily all referring to the same embodiment.

It is to be appreciated that the use of any of the following “/”,“and/or”, and “at least one of”, for example, in the cases of “A/B”, “Aand/or B” and “at least one of A and B”, is intended to encompass theselection of the first listed option (A) only, or the selection of thesecond listed option (B) only, or the selection of both options (A andB). As a further example, in the cases of “A, B, and/or C” and “at leastone of A, B, and C”, such phrasing is intended to encompass theselection of the first listed option (A) only, or the selection of thesecond listed option (B) only, or the selection of the third listedoption (C) only, or the selection of the first and the second listedoptions (A and B) only, or the selection of the first and third listedoptions (A and C) only, or the selection of the second and third listedoptions (B and C) only, or the selection of all three options (A and Band C). This may be extended, as readily apparent by one of ordinaryskill in this and related arts, for as many items listed.

Referring now to FIG. 5, a method for operating a low-crosstalk opticalswitch is shown. This embodiment operates without the phase errordetecting photodetector 302. Block 502 sets the phase of the phase tuner104 as needed for switching. Based on which phase—zero or pi—isselected, a different branch is used to work with a different heater. Inthe case of a phase of zero at the phase tuner 104, block 506 measurescrosstalk at a first photodetector 303. An initial acceptable thresholdis set low (e.g., at zero) to ensure that the loop is run at least once.If the crosstalk is below the acceptable threshold at block 508,processing returns to block 506. If crosstalk is above the threshold,block 510 uses the heaters 304 and 306 and loss compensator 106, ifimplemented as a loss tuner, to reduce the crosstalk. Block 512 thenadjusts the threshold accordingly to match the crosstalk value achievedby the adjustment.

In the case of a phase of pi at the phase tuner 104, block 514 measurescrosstalk at a second photodetector 303. If the crosstalk is below anacceptable threshold at block 516, processing returns to block 514. Ifcrosstalk is above the threshold, block 518 uses the heaters 304 and 306and loss compensator 106, if implemented as a loss tuner, to reduce thecrosstalk. Block 520 then adjusts the threshold accordingly.

Referring now to FIG. 6, a method for operating a low-crosstalk opticalswitch is shown. Block 602 sets the phase of the phase tuner 104 asneeded for switching. Based on which phase—zero or pi—is selected, adifferent branch is used to work with a different photodetector. In thecase of a phase of zero at the phase tuner 104, block 606 measures thephase error at the photodetector 302. The crosstalk is then measured ata first output photodetector 303 at block 607 and a first heater, either304 or 306, is activated at block 608. Block 610 determines whethercrosstalk has improved based on the measurement at the firstphotodetector 303. If crosstalk improved under the action of the firstheater, block 612 corrects the remaining phase error using the firstheater. Otherwise, block 614 corrects the remaining phase error with asecond heater, either 306 or 304.

In the case of a phase of pi at the phase tuner 104, block 616 measuresthe phase error at the photodetector 302. The crosstalk is then measuredat a second output photodetector 303 at block 618 and a first heater,either 304 or 306, is activated at block 620. Block 622 determineswhether crosstalk has improved based on the measurement at the secondphotodetector 303. If crosstalk improved under the action of the firstheather, block 612 corrects the remaining phase error with using thefirst heater. Otherwise, block 614 corrects the remaining phase errorwith the second heater, either 306 or 304.

Regardless of which heater is used, block 624 may optionally use theloss compensator 106. This step is only used if the loss compensator 106is implemented as a loss tuner rather than a static loss compensation.

Having described preferred embodiments of a low-crosstalkelectro-optical Mach Zehnder switch and method for operating the same(which are intended to be illustrative and not limiting), it is notedthat modifications and variations can be made by persons skilled in theart in light of the above teachings. It is therefore to be understoodthat changes may be made in the particular embodiments disclosed whichare within the scope of the invention as outlined by the appendedclaims. Having thus described aspects of the invention, with the detailsand particularity required by the patent laws, what is claimed anddesired protected by Letters Patent is set forth in the appended claims.

1. A method for optical switching, comprising: setting a phase shift ina first branch of an optical switch; setting a loss compensation in asecond branch of the optical switch to match a loss incurred on thefirst branch; measuring a crosstalk at an output of the optical switch;and if the measured crosstalk exceeds a crosstalk threshold, activatinga heater on the first or second branch to reduce the crosstalk.
 2. Themethod of claim 1, wherein the crosstalk is measured at one of twooutputs in accordance with the phase shift.
 3. The method of claim 1,wherein the first branch comprises a Mach-Zehnder phase shifter drivenin push-pull.
 4. The method of claim 3, further comprising measuring aphase error at an output of the Mach-Zehnder phase shifter.
 5. Themethod of claim 1, wherein activating the first or second heatercomprises selecting a heater to activate based on activating the firstheater and determining whether crosstalk improves.